Technical Abstract:
Sclerotinia trifoliorum causes stem and crown rot of chickpea and other leguminous forage crops. In order to study population structure and patterns of in-field spread, isolates of S. trifoliorum were intensively sampled from a chickpea field near Five Point, California, in 2006. All diseased plants resulted from crown infection. A total of 55 sclerotial isolates were obtained from 36 disease foci, and each isolate was obtained from a different diseased plant. The isolates were tested for mycelial compatibility groups (MCGs) and polymorphisms within the nuclear small subunit rDNA. Twenty-seven MCGs were found among the 55 isolates. The largest MCG contained 22 isolates, another MCG contained five isolates and three other MCGs each contained two isolates. The remaining 22 isolates each belonged to a different MCG. Three rDNA haplotypes were detected by analyzing four polymorphic sites in the rDNA. Isolates of the same MCG belonged to the same rDNA haplotype except one isolate. Although more than one MCG were found within disease foci, many of the isolates collected from adjacent plants belonged to the same MCG, suggesting that spreading by mycelial growth from adjacent plants is common. Also a single MCG was found dominating the population, suggesting that this MCG may be an early introduction or have a selective advantage in infecting chickpea plants.